I. Field of the Invention
This invention relates to a device for propagating signals between a continuous and a corrugated circular waveguide with minimized mismatch and low spurious mode excitations in two bands of frequency realized through a special inner boundary configuration in the transition which consists of dual-depth corrugations with changing dimensions along the length thereof.
II. Background Information
It is well known, satellite communication systems operate through the use of two distinct and well defined frequency bands where the higher frequency band (uplink) carries signals from the earth stations to the satellite while signals are sent from the satellite towards the earth stations in the lower frequency band (downlink). For such applications with certain stringent electrical specifications imposed on the radiation characteristics of the operating antennas, a corrugated horn feeding the reflector antenna system is considered to be one of the optimum solutions. This arrangement achieves satisfactory efficiency while maintaining low sidelobe and cross-polarized radiation levels.
With the introduction of the concept of frequency reuse where better utilization of the available frequency bands through simultaneous propagation of signals via two orthogonal polarizations at the same frequency is considered, the electrical specifications on the antenna characteristics have become furthermore stringent. In order to fulfil these requirements in terms of the cross-polarized radiation characteristics, often a dual-depth corrugated horn is employed which allows very low cross-polarized radiation characteristics to be maintained in two widely separated frequency bands, with an available freedom for adjustment of separation between the two bands.
However, for both the above mentioned applications utilizing a horn with conventional or dual-depth corrugations, the horn is conventionally connected at its throat region to a continuous circular waveguide which constitutes the common transmission line of the feed chain for the uplink as well as the downlink signals. The continuous circular waveguide supports the signals as the dominant TE11 mode. The arrangement calls for a transition to be devised to transform this mode into HE11 hybrid mode that propagates along the corrugated configuration of the horn. There are certain deleterious effects such as high return loss of the signals or unacceptable levels of spurious mode excitation that may accompany the transformation of TE11 to HE11 mode in the transition from a continuous circular waveguide to a corrugated circular waveguide, especially, when such transformation is desired at two widely separated frequency bands simultaneously.
In order that such a transition functions satisfactorily, a high susceptance boundary condition must be simulated near the continuous waveguide end through usage of appropriately configured corrugations which must gradually change their dimensions along the length of the transition to reach a low susceptance boundary condition at the other end where it connects into the horn. The manner of changing the corrugation configuration along the length of transition together with change in cross-section of the transition, is based on certain design criterion which prevents excitation of spurious modes or introduction of return loss at unacceptable levels.
Amongst the known transition for the transformation of TE11 to HE11 modes, there are two principal types which present satisfactory results for many applications. The first and most commonly used type of transition consists of a conventionally corrugated tapered circular waveguide transition where the depth of the corrugations are about half a free space wavelength deep at the highest frequency of operation at the continuous waveguide end, and starting with this value of depth of corrugations, they are diminished in depth gradually along the length of the transition such that about a quarter of a wavelength deep slot at the lowest frequency of operation is achieved at the end connecting into the horn. Such a transition operates with satisfactory electrical characteristics over a single and reasonably broad band. However, such a transition fails to operate satisfactorily when optimized performance is desired in two widely separated bands. The second and the rather involved, in terms of its manufacturing, type of the transition consists of a tapered circular waveguide transition furnished with a special corrugated boundary made of ring loaded corrugations. These ring loaded corrugations have a wider opening at the bottom to achieve broadened band of operation that encompasses the widely separated bands.
In terms of manufacturing, due to the unusual shape of the corrugations, the ring loaded corrugation configuration presents many difficulties. Since conventional machining techniques cannot be used to make such corrugations, they must be either configured with discs or electroformed on a mandrel which is later removed by chemical dissolving. Needless to emphasize, such methods of manufacturing call for considerable amount of effort and cost in production. Of course, in terms of the electrical performance, this second type of transition can potentially achieve the desired specification far more satisfactorily than the first type discussed before.